linux-kernel - Re: [PATCH 03/32] locking/lockdep: lockdep_set_no_check

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Message-ID: <ZFy2JqL/xkvIn+O1@moria.home.lan>
Date:   Thu, 11 May 2023 05:32:22 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 03/32] locking/lockdep: lockdep_set_no_check_recursion()

On Thu, May 11, 2023 at 10:25:44AM +0200, Peter Zijlstra wrote:
> On Wed, May 10, 2023 at 04:38:15PM -0400, Kent Overstreet wrote:
> > On Wed, May 10, 2023 at 10:59:05AM +0200, Peter Zijlstra wrote:
> 
> > > Have you read the ww_mutex code? If not, please do so, it does similar
> > > things.
> > > 
> > > The way it gets around the self-nesting check is by using the nest_lock
> > > annotation, the acquire context itself also has a dep_map for this
> > > purpose.
> > 
> > This might work.
> > 
> > I was confused for a good bit when reading tho code to figure out how
> > it works - nest_lock seems to be a pretty bad name, it's really not a
> > lock. acquire_ctx?
> 
> That's just how ww_mutex uses it, the annotation itself comes from
> mm_take_all_locks() where mm->mmap_lock (the lock formerly known as
> mmap_sem) is used to serialize multi acquisition of vma locks.
> 
> That is, no other code takes multiple vma locks (be it i_mmap_rwsem or
> anonvma->root->rwsem) in any order. These locks nest inside mmap_lock
> and therefore by holding mmap_lock you serialize the whole thing and can
> take them in any order you like.
> 
> Perhaps, now, all these many years later another name would've made more
> sense, but I don't think it's worth the hassle of the tree-wide rename
> (there's a few other users since).

Thanks for the history lesson :)